Sugarcane chopper harvesters use airflow devices to generate a flow of air to separate the extraneous matter, such as leaves and dirt from the chopped sugarcane, and expel it from the flow of chopped cane. This separation occurs in what is known as a cleaning chamber. 
The most popular type of cleaning chamber has an axial flow fan, referred to as an extractor, as the air-generating device. The fan is mounted above the flow of chopped material and the unwanted material passes through the fan itself. The shape of the cleaning chamber is round to match the shape of the fan. The material to be separated is introduced into the cleaning chamber from one side and usually within two feet of the fan. At this distance from the fan, the air velocity is highest at the tips of the blades with ever-decreasing velocity towards the fan's center. Most of the extraneous matter is removed as it travels through the highest velocity areas. This occurs when the material enters the chamber and again when it reaches the far side of the chamber. 
The design of the fan itself is composed of a hub and a number of fan blades that attach to the hub. There are many designs of fans. Some have small hubs with long blades and others have large hubs with short blades. In either case, a dead zone area exists directly below the hub such that the area in the cleaning chamber directly below the hub has little air movement. The dead zone area can be quite a bit larger than the actual hub diameter in some cases. Where a multitude of fans attach to a small hub, an over-lapping or almost over-lapping of blades occurs. Very little air movement occurs in this over-lapped area and little if any extraneous matter is withdrawn through this area. As an example, a popular state of the art 57″ tip diameter fan with four blades and an 11″ hub diameter has an effective blade over-lap diameter of 24″. 
In recent years, in an attempt to liven the dead zone, nose cone-shaped  devices have been added to the fan hub. The fan efficiency is increased by adding these devices such that the same amount of air can be moved at a slower fan RPM. These devices do help nullify the dead zone effect by decreasing the dead zone to a point; however, the largest percentage of leaf and extraneous trash is removed while it is moving through the outer-most high-speed annular ring of air. Once through the high-speed air area, an air speed is eventually encountered that is not sufficient enough to levitate the material, and no further cleaning is achieved until the material reaches the opposite side of the cleaning chamber and is subjected to the high-speed flow again. 
The problem to be solved is that of keeping the material exposed to the high-velocity air located at the tips of the fan blades and/or to increase the airflow in the center of the fan. 